Change windows installer language code to en_GB

[ann0see]

    `en_UK`  is not the correct language code for English as used in Great Britain, by the way.  See [Wiki](https://en.wikipedia.org/wiki/IETF_language_tag#ISO_3166-1_and_UN_M.49), [language-territory for en](https://unicode-org.github.io/cldr-staging/charts/37/supplemental/language_territory_information.html#en), [territory-language for GB](https://unicode-org.github.io/cldr-staging/charts/37/supplemental/territory_language_information.html#GB), [language-territory for uk](https://unicode-org.github.io/cldr-staging/charts/37/supplemental/language_territory_information.html#uk) and [territory-language for UA](https://unicode-org.github.io/cldr-staging/charts/37/supplemental/territory_language_information.html#UA)

Originally posted by @pljones in #2880 (comment)